ARM: 8794/1: uaccess: Prevent speculative use of the current addr_limit

Julien Thierry · Russell King · commit 621afc677465 · 2018-10-05T10:51:15.000+01:00
A mispredicted conditional call to set_fs could result in the wrong addr_limit being forwarded under speculation to a subsequent access_ok check, potentially forming part of a spectre-v1 attack using uaccess routines. This patch prevents this forwarding from taking place, but putting heavy barriers in set_fs after writing the addr_limit. Porting commit c2f0ad4 ("arm64: uaccess: Prevent speculative use of the current addr_limit"). Signed-off-by: Julien Thierry <julien.thierry@arm.com> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
diff --git a/arch/arm/include/asm/uaccess.h b/arch/arm/include/asm/uaccess.h
@@ -69,6 +69,14 @@ extern int __put_user_bad(void);
 static inline void set_fs(mm_segment_t fs)
 {
 	current_thread_info()->addr_limit = fs;
+
+	/*
+	 * Prevent a mispredicted conditional call to set_fs from forwarding
+	 * the wrong address limit to access_ok under speculation.
+	 */
+	dsb(nsh);
+	isb();
+
 	modify_domain(DOMAIN_KERNEL, fs ? DOMAIN_CLIENT : DOMAIN_MANAGER);
 }
 

Original file line number	Diff line number	Diff line change
`@@ -69,6 +69,14 @@ extern int __put_user_bad(void);`
`69`	`69`	`static inline void set_fs(mm_segment_t fs)`
`70`	`70`	`{`
`71`	`71`	`current_thread_info()->addr_limit = fs;`
	`72`	`+`
	`73`	`+ /*`
	`74`	`+ * Prevent a mispredicted conditional call to set_fs from forwarding`
	`75`	`+ * the wrong address limit to access_ok under speculation.`
	`76`	`+ */`
	`77`	`+ dsb(nsh);`
	`78`	`+ isb();`
	`79`	`+`
`72`	`80`	`modify_domain(DOMAIN_KERNEL, fs ? DOMAIN_CLIENT : DOMAIN_MANAGER);`
`73`	`81`	`}`
`74`	`82`